qmake: let QMakeVfs::readFile() report ENOFILE explicitly

when the QFile object is already constructed, querying whether the file
exists is actually cheap, so do it right away instead of later on
demand. that makes the calling code a bit cleaner.

fwiw, that we need to explicitly query the file's existence at all is a
result of QFile's completely useless error "codes" (which merely say
which function failed, as if the caller would not know).

Change-Id: Ifec39d05b1713d8128046f679287e510f10e45dc
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>
(cherry picked from qtcreator/5ba32e3484ead2e35cc7732dcd59a97e7459dbfd)
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>